Many analog and digital circuits require multiple voltage or current levels to operate a system. Portable systems are often limited to using batteries to supply power, with the limitation that batteries each only supply one voltage level. DC to DC switchers are then employed to generate the remaining voltage levels.
DC to DC switchers are often the preferred solution over voltage or current regulators because switcher efficiencies exceed regulator efficiencies by a considerable margin. However, switchers do not operate as constant current sources.
DC to DC switchers may comprise an integrated circuit (IC) and associated external electronic components to operate as a power supply, such as a switching transistor, resistors, inductors and capacitors. The switching integrated circuit causes the switching transistor to rapidly open and close, allowing current to flow through the components at the appropriate amounts in order to charge an output capacitor or other component which supplies a constant voltage to a load. There are several common topologies, or configurations, for the external components normally associated with a DC to DC switcher, such as boost, buck, or buck-boost. These common topologies each have advantages for different applications of the power supply, but each operates as a constant voltage source.
DC to DC switchers maintain a constant output voltage by monitoring the output voltage and adjusting the switching transistor "ON" time to keep the output capacitor fully charged under varying load conditions. The switching transistor is switched on and off rapidly enough that the voltage level stored in the output capacitor appears to the load to remain at a constant level, much as a fluorescent light turns on and off rapidly enough that it appears to be at a constant light level to the eye. The switching transistor can be adjusted to remain on a large percentage of the time to supply a higher output voltage or a small percentage of the time to supply a lower output voltage. The DC to DC switcher employs a feedback mechanism which monitors the output voltage, generally via a resistor divider network, which allows the DC to DC switcher to detect whether the output voltage needs to be boosted or allowed to fall. This voltage fed back into the DC to DC switcher from the output is called a feedback voltage, and is generally required to be set at 1.2 to 1.5 volts depending on a reference voltage value set internally by the device manufacturer. The feedback voltage can be fixed at the required level using the resistor divider to cut the desired output voltage down to the required feedback voltage level.
This feedback mechanism does not support current control, as it is designed to monitor a voltage and requires a voltage level as the feedback input to the DC to DC switcher.
A need therefore exists for a comprehensive system and method enabling a power supply based on a DC to DC switcher to employ a feedback mechanism enabling control of the output current in order to supply a constant current.
A further need exists to provide a system and method for supplying a constant current using a DC to DC switcher which may be employed in various switcher circuit topologies without significant change.
An additional need exists to provide a system and method for supplying a constant and/or controllably adjustable current using a DC to DC switcher with an external control signal to set the desired current level, wherein the external control signal may be either a DC signal or a pulse width modulated (PWM) signal.